Q: manage space on my iphone 4s

i have an iphone 4s with 16GB of space.  When I check the usage under settings, it says that I have only 470 MB available space left, and that I am using 13.2GB. My first question is, what happened to the other 2+GB, is that all used by the OS?  My second question which is more important, is that when I add up all of the space used by all of the applications listed under the Usage area within Settings, it only adds up to about 7.5 GB.  How can I find out what is using the other 5.7 GB of space?

One thing that I do not see listed under applications is Photos.